Transfer Switch Load Calculator

ANA›Life Services Authority›National Calculator Authority›Transfer Switch Load Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Transfer Switch Load Calculator

Calculate total connected load, transfer switch ampacity, and recommended generator size based on your electrical loads.

System Voltage (V)

120V (Single Phase) 240V (Single Phase) 208V (3-Phase) 480V (3-Phase)

Phase Configuration

Single Phase Three Phase

Power Factor

Demand Factor (%)

### Load Entries

Load Description Watts (W) Quantity

Calculate Transfer Switch Load

Total Connected Load

Demand Load

Required Transfer Switch

Recommended Generator

(function() { // Default loads var defaultLoads = [ ["HVAC / Air Conditioner", 3500, 1], ["Refrigerator", 800, 1], ["Lighting (Essential)", 600, 1], ["Sump Pump", 1200, 1], ["Well Pump", 1500, 1] ];

window.traAddRow = function(desc, watts, qty) { var container = document.getElementById("tra-rows"); var idx = container.children.length; var row = document.createElement("div"); row.className = "calc-row"; row.style.marginBottom = "6px"; row.innerHTML = '' + '' + '' + '✕'; container.appendChild(row); };

window.traRemoveRow = function(btn) { btn.closest(".calc-row").remove(); };

// Init default rows defaultLoads.forEach(function(l) { traAddRow(l[0], l[1], l[2]); });

window.traCalc = function() { var errEl = document.getElementById("tra-error"); var resEl = document.getElementById("tra-result"); errEl.style.display = "none"; resEl.style.display = "none";

var voltage = parseFloat(document.getElementById("tra-voltage").value); var phase = parseInt(document.getElementById("tra-phase").value); var pf = parseFloat(document.getElementById("tra-pf").value); var demand = parseFloat(document.getElementById("tra-demand").value) / 100;

// Validate globals if (isNaN(voltage) || voltage 1.0) { showErr("Power factor must be between 0.10 and 1.00."); return; } if (isNaN(demand) || demand 1) { showErr("Demand factor must be between 1% and 100%."); return; }

// Collect rows var rows = document.getElementById("tra-rows").children; if (rows.length === 0) { showErr("Add at least one load."); return; }

var totalW = 0; var loadLines = []; var hasError = false;

for (var i = 0; i = switchA; }) || Math.ceil(switchA/100)*100;

// Generator sizing: kVA = demandVA / 1000, add 25% headroom var genKVA = (demandVA / 1000) * 1.25; var genKW = genKVA * pf;

// Standard generator sizes (kW) var stdGen = [5,7.5,10,12,15,17.5,20,22,25,30,35,40,45,50,60,75,100,125,150,200]; var recGen = stdGen.find(function(g){ return g >= genKW; }) || Math.ceil(genKW/10)*10;

// ── Display ──────────────────────────────────────────────────── document.getElementById("tra-total-w").textContent = totalW.toLocaleString() + " W"; document.getElementById("tra-demand-w").textContent = demandW.toLocaleString(undefined,{maximumFractionDigits:0}) + " W"; document.getElementById("tra-switch-a").textContent = recSwitch + " A"; document.getElementById("tra-gen-kva").textContent = recGen + " kW";

// Detail breakdown var rows_html = loadLines.map(function(l){ return '' + l.desc + '' + l.w.toLocaleString() + ' W × ' + l.qty + '' + l.lineW.toLocaleString() + ' W'; }).join("");

document.getElementById("tra-detail").innerHTML = '' + 'LoadUnit × QtySubtotal' + '' + rows_html + '' + 'Total Connected Load' + totalW.toLocaleString() + ' W' + '' + '' + 'Demand Load: ' + totalW.toLocaleString() + ' W × ' + (demand*100).toFixed(0) + '% = ' + demandW.toLocaleString(undefined,{maximumFractionDigits:0}) + ' W' + 'Apparent Power: ' + demandW.toLocaleString(undefined,{maximumFractionDigits:0}) + ' W ÷ ' + pf + ' PF = ' + demandVA.toLocaleString(undefined,{maximumFractionDigits:0}) + ' VA' + 'Load Current: ' + currentA.toFixed(2) + ' A' + 'Switch Current (×1.25): ' + switchA.toFixed(2) + ' A → ' + recSwitch + ' A' + 'Generator kVA: ' + (demandVA/1000).toFixed(2) + ' kVA × 1.25 = ' + genKVA.toFixed(2) + ' kVA' + 'Generator kW: ' + genKW.toFixed(2) + ' kW → ' + recGen + ' kW' + '';

document.getElementById("tra-nec").innerHTML = 'NEC References: NEC 230.95 (transfer equipment), NEC 210.20 (125% continuous load factor), ' + 'NEC 445.13 (generator ampacity), NEC 702.5 (optional standby systems). ' + 'Transfer switch must be rated for the full available fault current at the point of installation.';

resEl.style.display = "block"; };

function showErr(msg) { var el = document.getElementById("tra-error"); el.textContent = msg; el.style.display = "block"; } })();

#### Formulas Used

Total Connected Load (W): Σ (Watts × Quantity) for all loads

Demand Load (W): Total Load × Demand Factor

Apparent Power (VA): Demand Load (W) ÷ Power Factor

Load Current — Single Phase (A): VA ÷ V

Load Current — Three Phase (A): VA ÷ (√3 × V)

Transfer Switch Rating (A): Load Current × 1.25 (NEC 125% continuous load factor) → rounded up to next standard size

Generator Size (kW): (VA ÷ 1000) × 1.25 headroom × Power Factor → rounded up to next standard size

#### Assumptions & References

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References

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